Delay line anodes are apparata used in time- and/or position-sensitive detectors to encode the time and position of impact of particles (e.g., electrons, atoms, ions, molecular complexes, etc.) and/or photons incident upon the detector. The function and operation of delay line anodes in such detectors is briefly described here to provide the reader with a better understanding of the background of delay line anodes, but if further background information is needed, the reader is referred to the discussions in U.S. Pat. Nos. 4,431,921 and 4,965,861 to Filthuth, and U.S. Pat. No. 5,644,128 to Wollnik et al.
Prior to further discussion of delay line anodes, it is initially important to note that this document generally refers to several delay line detectors (and other detectors) as “anodes,” which implies that these devices generate signals by collecting electrons. In typical practice, this is indeed the case. However, such detectors have also been used in arrangements whereby signals are introduced into the delay line via an electromagnetic pulse (EMP). As there is no transfer of electrons in this case, the delay line detector is not technically an anode. However, for simplicity, this document uses the term “anode” to refer to both cases because the fundamental operating principles are similar.
In typical time- and/or position-sensitive detectors, exemplified by the arrangement shown in FIG. 1, the particle or photon of interest (“PoI”) incident upon the detector 100—the PoI being depicted by its flight path 102—is first converted into a relatively large number of electrons (generally approximately 102–108 electrons, depicted at 104) via an electron amplification device (EAD) 106. The EAD 106 is typically a microchannel plate (MP), microsphere plate (MSP), or gas electron multiplier (GEM), though other means of electron amplification are possible. The centroid of the electron cloud 104 generated by the EAD 106 corresponds to the position at which the PoI 102 struck the EAD 106. Most EAD 106 are insensitive to photons, and therefore in photon detection applications, a photocathode such as cesium iodide is often placed between the incident photon 102 and the EAD 106 to convert the photon 102 into electrons that impact the EAD 106 and trigger the amplification event. The electron cloud 104 generated by the EAD 106 is then driven via a bias voltage to one or more delay line anodes, with two such anodes being depicted in FIG. 1 by upper delay line anode 108 and lower delay line anode 110, for encoding of the time and position of impact of the electron cloud 104 on the delay line anode(s).
A delay line anode typically has a delay line, i.e., a conducting signal line coupled with a ground line, arrayed over the imaging or active area of the delay line anode in a manner such that a position or coordinate in the dimension of interest corresponds to a distance or length along the delay line. Generally, the relationship between the position/coordinate in the dimension of interest and the distance along the delay line is linear, but other relationships (e.g. nonlinear, radial, spiral) can be implemented. Each dimension for which position is to be determined generally requires a minimum of one delay line anode. Accordingly, two delay line anodes may be used in order to encode position in two dimensions, as in FIG. 1, wherein anode 108 encodes position in the Y direction and anode 110 encodes position in the X direction. Additional anodes can be used to provide redundant information for double-hit encoding (i.e., where subsequent electron impacts may occur before signals from a prior electron impact leave an anode) and for other purposes.
When the electron cloud 104 impacts the delay line anode 108 or 110 (directly or via other means), an electromagnetic signal is induced in the delay line anode, and it propagates as two distinct pulses (EMP), one towards each end of the anode's delay line. Each end of the delay line is connected to a series of timing components 112 whose function is to determine the arrival time of the EMP at that end, such that the arrival times of the two EMP at opposite ends of the delay line are independently measured and encoded. Several timing components are typical. Initially, each end of the delay line is often connected to a high-speed, low-noise preamplifier that amplifies the EMP (generally by a factor of 10×–100×). The preamplifier is often followed by a pulse shaper which modifies the EMP into a more easily monitored timing pulse, with such a pulse shaper being exemplified by a constant fraction discriminator (CFD). A CFD converts the near-gaussian-shaped EMP of varying amplitude into a sharp timing pulse that corresponds to the arrival time of the pulse at the end of the delay line. The pulse shaper is then often followed by a time-to-digital converter (TDC) that measures the arrival time of the timing pulse. Other components may be included in the timing components 112 as well, but the relevant physical relationship is that the difference in the arrival times of the two EMP at the opposite ends of a given delay line corresponds to the time and position of impact of the original event, i.e., to the impingement of the electron cloud on the delay line anode. The time and position of impact is in turn used to calculate desired experimental quantities such as the mass of the PoI.
Detector performance is characterized by the speed, accuracy, and precision with which the detector interprets impact events. Performance is limited by properties inherent in the EAD 106, the delay line anode(s) 108 and 110, the timing components 112, and in the interaction between these components. The limiting factor for resolution of a delay line anode detector is typically in the timing components 112, e.g., in pulse shaper accuracy or the accuracy with which the pulse shaper converts the amplified EMP from the signal line into a timing pulse. Pulse shaper accuracy is in turn dependent upon size and shape distributions of input EMP. Input EMP that are large and sharp minimize inaccuracies that may arise from pulse voltage measurement errors. Further, consistent input EMP size and shape allows tuning of a pulse shaper to a particular pulse shape, which also improves the accuracy of the pulse shaper and the detector.
Typically, preamplifiers, pulse shapers, and/or other timing components are custom designed and fabricated according to the electrical properties of the delay line anode(s) with which they will be used. Alternatively, prefabricated high performance preamplifiers and pulse shapers may be tuned or adjusted to optimize performance with the delay line anode(s) with which they will be used. In order to minimize the time and cost of achieving a satisfactory match between timing components to the delay line anode(s) of the detector, it is desirable to employ a fabrication technique that produces a delay line anode with consistent properties such that timing components can be designed for mass production and fabricated in quantity, rather than being designed, fabricated, and/or tuned on an anode-by-anode basis.
Apart from the timing components, the delay line anode can affect detector accuracy by affecting the size and sharpness of the input EMP into the timing components through attenuation, and also by impedance mismatch between the anode and the timing components. Attenuation, the amount of energy lost per unit length of delay line traveled by an EMP, is a fundamental property of a transmission line that is dependent upon numerous factors (such as the dimensions of the signal and ground lines, the DC resistance of the signal line, loss properties of the dielectric layer between the signal and ground lines of the delay line, and the thickness of the dielectric). For any given length of the delay line, there is a tradeoff between minimizing attenuation and maximizing resolution. Longer delay line length means that the EMP travel for a longer period of time in the delay line, which results in better resolution for a given timing accuracy since the differences in EMP arrival times at the ends of the delay lines will be greater. However, for a given active area on the delay line anode, longer delay line length increases attenuation in two ways: the longer distance traveled in the delay line attenuates EMP, and additionally the width of the delay line must be made smaller (thereby increasing attenuation) in order to accommodate the added delay line length within the same active area. Conversely, shorter delay line length has the advantage of less attenuation of EMP, and additionally a detector that employs anodes with shorter delay lines can record more events in a given time since the EMP exit the delay line in a shorter time. However, the shorter the delay lines, the less difference in the arrival times of the EMP at opposite ends of the delay line, and therefore the timing components must be more accurate in order to obtain the same resolution as a detector employing longer delay lines.
As previously noted, it is also desirable to minimize impedance mismatch between the delay line anode and the timing components (and also between the timing components themselves). Impedance mismatch reduces the speed at which the detector can record events because mismatch causes reflections of the EMP at the point of mismatch. The reflected portion of the EMP can propagate back through the detector, thereby increasing the time before EMP clears the detector, thereby slowing the maximum rate at which the detector can record events (unless complex algorithms are used to interpret EMP arrivals, which is preferably avoided). Similarly, unless precautions are taken, the reflected EMP may create a spurious timing event at the other end of the delay line.
It is believed that an arrangement such as that shown in FIG. 1, wherein a detector utilizes multiple delay line anodes having delay lines arrayed in serpentine layers, was first proposed by Siegmund et al. (“A high resolution delay line readout for microchannel plates,” EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy and Atomic Physics, Proc. SPIE 1159, 476–485 (1989)). Advantages of this multilayer serpentine delay line detector arrangement are high resolution over a relatively large area, high throughput rate, good spatial linearity, simplicity, reliability, low power requirements, and a robust design. Friedman et al. (“Multilayer anode with crossed serpentine delay lines for high spatial resolution readout of microchannel plate detector,” Rev. Sci. Instrum. 67(2), 596–608, February 1996) describe one possible scheme and fabrication method for a detector arrangement of this type, wherein two serpentine microstrip delay line anodes are used to encode positional information in two dimensions. Of particular interest is the method proposed for fabrication of the detector, which will now be described with reference to FIG. 2. The two anodes of the detector are fabricated separately using commercially developed methods for microstrip printed circuit board fabrication. Each of the two anodes begins as a double-sided, copper-clad, RT/DUROID 6010 ceramic-filled PTFE dielectric board (Rogers Corp., Chandler, Ariz.). On the upper side of each board, copper is etched away using standard photolithography techniques to leave only a single serpentine signal line with solder pads at its ends. The lower side of each board is the ground plane (ground line) for the signal line situated on the upper side of that board. As will be discussed below, the boards/anodes will be bonded together in generally coplanar relationship, and thus the foregoing arrangement is shown in FIG. 2 with one board (the upper anode) of the detector 200 shown at 202, with the upper anode 200 including upper signal line 204 and upper ground line 206 spaced by the board's dielectric material 208. The upper ground line 206 is etched so that it extends in a serpentine pattern in parallel spaced alignment with the upper signal line 204. Similarly, the other board (the lower anode) is depicted as 210, and includes lower signal line 212 and lower ground plane 214 spaced by the lower board's dielectric material 216. Here, the lower ground plane 214 may be (but need not be) etched to mirror the lower signal line 212. Note that in FIG. 2, the signal lines 204 and 212 are oriented in generally perpendicular directions, as in FIG. 1, and thus the upper delay line (the upper signal line 204 and upper ground line 206) is shown with lengths extending out of the drawing toward the viewer, while the lower delay line (the lower) signal line 212 extends in a perpendicular direction and thus has lengths running parallel to the plane of the drawing. (The lower ground place 214 may also extend in this direction if etched to have such an orientation.)
The contiguous, multilayer anode array is made by bonding the two anodes 202 and 210 together with a back plate 218 using BONDPLY bonding agent, depicted in layers at 220 and 222. The BONDPLY bonding agent is a less dense/more porous form of RT/DUROID 6002 ceramic- and glass-filled PTFE dielectric, and under heating to 385° C. under 1700 psi pressure, it converts into RT/DUROID 6002 material and thereby fuses adjacent materials that have appropriately treated surfaces. This bonding is done in two steps. First, the back plate 218 is bonded to the lower ground plane 214 of the lower anode 210 at bonding agent layer 220, then the lower signal line 212 layer of the lower anode 210 is bonded to the upper ground line 206 of the upper anode 202 at bonding agent layer 222. The upper bonding agent layer 222 must generally be thick in order to diminish crosstalk between the top and bottom anodes 202 and 210.
During the foregoing process, the upper anode dielectric layer 208 is continuous. However, proper functioning of the delay line readouts requires removal of the dielectric material 208 bounding the upper delay line (e.g., between the segments of the upper signal line 204 and upper ground line 206), as well as the portions of the dielectric upper bonding agent layer 222 adjacent the upper delay line, to expose the lower signal line 212 and thereby allow electrons from the electron cloud to reach the lower signal line 212. Such removal is generally performed by laser ablation machining. The resulting detector resembles the one illustrated in FIG. 1, though the illustrated upper and lower delay lines 108 and 110 are not an entirely accurate depiction, particularly in that they generally do not have all excess dielectric removed as in FIG. 1 and are generally provided with some form of support structure. Most particularly, the lower delay line 110 is often provided in a continuous circuit board rather than in the freestanding serpentine array shown.
A primary disadvantage of this fabrication method is the need to remove multiple layers of dielectric material 208 and 222 by laser ablation machining in order to expose the lower anode 210 and its signal line 212. This machining is difficult and expensive, particularly owing to the high aspect ratio (ratio of depth to width)—approximately 4:1 —of the channels of dielectric that are ablated. The need for deep cutting significantly heats the board, often resulting in warping and imperfect production unless cutting is done in multiple stages, which greatly increases manufacturing time and cost.
Apart from problems with the fabrication method, there are numerous disadvantages with the structural features of anodes made by the foregoing method. First, the resulting anode array detector is often incompatible with Ultra High Vacuum (UHV) environments. The detector is fabricated using standard printed circuit board techniques and materials, but standard circuit board materials are incompatible with UHV environments due to insufficient temperature stability and outgassing. Most typical printed circuit board materials are stable at temperatures up to approximately 120° C.; however, UHV environments typically require that materials be stable to approximately 150°–200° C. In addition, most adhesives and circuit board materials used in traditional printed circuit board manufacture continuously release gas, and this is similarly unsuitable for use in UHV environments.
Second, the need to bond the two anodes/boards 202 and 210 together results in undesirable nonuniformity in detector properties. Variation in the thickness of the upper bonding agent layer 222, and/or variation in distance between signal lines 204 and 212, will affect the impedance of both of signal lines 204 and 212. In particular, owing to the presence of the upper ground line 206, the thickness of the upper bonding agent layer 222 strongly affects the electrical properties of the lower signal line 212. In general (but depending on the bonding materials used), the thinner the upper bonding agent layer 222, the slower the EMP propagation speed, the higher the attenuation, and the lower the impedance. The effects of variation in bond thickness/distance are further amplified by the fact that the bonding agent layer 222 has a relatively high dielectric constant (generally having a relative dielectric coefficient of approximately 4). As a result, the impedance of the lower signal line 212 will be difficult to accurately control, thus causing problems with impedance matching to the timing components 112 and other components. As one might expect, if the upper and lower signal lines 204 and 212 display different performance characteristics—as they almost inevitably will do—the complexity of the timing components 112 grows and results become more difficult to interpret. Therefore, to achieve maximum performance, the electronics and/or other components for the upper and lower signal lines 204 and 212 must be fabricated to different specifications or, at a minimum, must be tuned differently. This precludes efficient mass production of a complete detector assembly.
Third, the permanence of the bond between the two anodes 204 and 210 is also disadvantageous in that if one anode is found to be defective after bonding to the other anode, the entire detector must be scrapped. This potentially results in discarding a non-defective anode along with the defective one, thereby increasing losses from wasted time and materials.
Fourth, detector accuracy is partially dependent on alignment of the two delay lines in a known orientation—typically 90°—but the nature of the bonding process used to bond the upper and lower anodes 202 and 210 together makes precise positional registration between their delay lines difficult to achieve. Heat and pressure are required to activate the bonding function of the upper bonding agent layer 222, and shifting due to flow of the bonding agent between the upper and lower anodes 202 and 210 may occur under these conditions. Further, since bonding renders the anode alignment permanent, deviations from the desired alignment cannot be corrected after the bonding is completed.
Fifth, the active area of a standard printed circuit board anode (i.e., the area over which the delay line receives particles) is practically limited by the resistance of delay lines having widths achievable with standard printed circuit board fabrication methods, which is typically 60 microns or less. If delay lines are thinner, more may be fit within any given active area; however, thinner lines have higher electrical resistance, and thus can exhibit undesirable characteristics (in particular higher attenuation).
Sixth, anodes fabricated using standard printed circuit board techniques are generally limited to a planar form. If it is desirable to have a nonplanar or otherwise irregularly-shaped anode, or an anode with changeable configuration, the foregoing scheme seems inapplicable as a practical matter because the etching, bonding, ablation, etc. are difficult to economically perform on non-planar boards.
Seventh, the foregoing fabrication scheme gives rise to limitations in the resulting anode. Standard printed circuit board manufacturing techniques etch the desired pattern into the conductor from the top/exposed side of the conductor, and thus the width of the etched signal line 204 or 212 is smaller at the top surface of the board 202 or 210 than at the bottom of its signal line (the portion bonded to the dielectric layer 208 or 216). The resulting signal lines have a trapezoidal cross-section with a narrow top and wider base, and more DC resistance for a given maximum width than a signal line of non-varying width.
There have been efforts to develop anodes and fabrication methods which avoid the foregoing problems. One fabrication method eliminates the difficult and costly laser ablation machining by changing the form of the upper delay line anode. In this scheme, the lower delay line anode is not exposed—its signal line remains “buried” beneath the upper delay line anode and the intermediate bonding layer—and the signal generated by the incident electron cloud is transferred to the lower signal line from the upper anode by conductive leads called vias. Although this scheme eliminates the problems of laser ablation, the vias add cost and complexity to the fabrication process. Further, each via adds a capacitive load to the signal line, thus significantly altering its electrical properties (generally decreasing impedance and pulse amplitude, and slowing propagation speed). Finally, the vias act as miniature antennae between the two signal lines, increasing crosstalk between lines and dissipation of the signal, which ultimately decreases the overall performance of the anodes. The vias also cause the bottom signal line to display significantly different electrical and performance characteristics compared with the top signal line, thereby reducing the possibility that timing and other components having the same characteristics can be used for the top and bottom anodes.
To summarize, prior delay line anode fabrication methods are difficult and costly to execute, and they yield multi-anode detectors wherein the component anodes display significantly different electrical characteristics, thereby complicating electronics and interconnection issues and ultimately limiting detector performance.